Understand open PEN conductor fault protection

Helpful video reference. John Ward (jwflame) covers earthing systems, EV charging connection options and open PEN detection devices in detail in "Earthing systems, EV charging connection options and open PEN detection devices". John is one of the most thorough electrical educators on YouTube and gets the UK-specific earthing terminology right throughout.

1. What a TN-C-S (PME) supply is

Most UK homes, somewhere around 85%, are connected via a PME supply. PME stands for Protective Multiple Earth. The Distribution Network Operator (DNO) runs a single combined cable from the substation to your property. That cable carries the line conductor and a combined neutral-and-earth conductor called the PEN (Protective Earth Neutral).

At your meter position, the PEN terminates and splits into two separate paths inside your installation: the neutral bar and the main earth terminal (MET). The green-and-yellow earth wires throughout your house ultimately connect back to the MET, which is bonded to the DNO's neutral via that PEN conductor.

The alternative is TN-S earthing, where a separate earth cable runs alongside the neutral from the substation. TN-S is common in older properties and areas with older street cabling, though many apparent TN-S systems have already been converted to PME by the DNO during network upgrades.

2. What happens when the PEN conductor breaks

A PEN conductor can fail for several reasons: corrosion in a cable joint, mechanical damage during groundworks, or a fault inside the DNO's street distribution network. When it breaks, two things happen simultaneously.

First, the neutral return path for your single-phase supply is broken. Lights and appliances on the affected phase may go dim or bright as the neutral floats -- a symptom sometimes called a lost neutral rather than an open PEN, though they are related.

Second, and more dangerously, your MET is no longer connected to the DNO's earth reference. Your earth is now floating. Load currents flowing through neighbouring properties on the same distribution cable can cause a voltage to appear across the break. In a worst case, your MET -- and everything bonded to it -- can rise to a substantial fraction of line voltage. Gas pipes, water pipes, the consumer unit enclosure and the case of any EV charger can all become live relative to true earth (the ground beneath your feet).

3. Who is at risk from an open PEN fault

Indoor metalwork is dangerous, but the highest risk is outdoors. A person standing on damp ground is in direct contact with true earth potential. If they simultaneously touch metalwork that is connected to a risen MET, the voltage difference appears across their body.

EV chargers are the classic example. The charger case, the charge cable and the vehicle bodywork are all earthed back to the MET. A user unplugging the cable while standing on damp concrete, bare grass or wet paving could receive a shock if the PEN has broken upstream.

Hot tubs, outdoor sockets, metal garden structures, metallic water pipes and outdoor taps carry the same risk. Anything earthed and accessible from outside the building needs careful thought on a PME supply.

4. What BS 7671 Section 722 requires

BS 7671 Amendment 1, which came into force in July 2022, added mandatory requirements under Section 722 for electric vehicle charging equipment. Regulation 722.411.4.1 specifically addresses installations on TN-C-S (PME) supplies.

The regulation requires that the EV charger circuit includes protection against the risk of an open PEN conductor. The protection must automatically disconnect the EV charger if a PEN fault is detected, before anyone can be exposed to the risen earth potential.

This requirement applies to all new domestic EV charger installations from the date the amendment came into force. Existing installations fitted before Amendment 1 should be reviewed -- while they are not automatically illegal, they do not meet current best practice. If you had a charger installed a few years ago without any earth electrode or open PEN device, it is worth asking an electrician to check.

5. Three ways to meet the requirement

Earth electrode plus open PEN detection relay. An earth electrode (typically a copper-clad steel rod driven into the ground outside) gives the circuit a local earth reference independent of the DNO's PEN. An open PEN detection relay monitors the voltage between your MET and the electrode. If the PEN breaks and the MET rises, the voltage difference exceeds a threshold -- usually around 50 V -- and the relay opens, cutting supply to the charger. This is the most common retrofit solution and works with any charger.

Combined connection unit. Products such as the Matt:e Connection Unit combine the earth electrode connection, the PEN fault detector, a Type A RCD and the supply to the charger in a single wall-mounted enclosure. They are popular because they keep compliance simple and provide a clear audit trail on the installation certificate.

Smart charger with internal PEN fault detection. Some manufacturers have integrated PEN fault detection into the charger itself. The charger monitors for earth faults internally and disconnects if a fault is detected. If your installer proposes this route, ask them to confirm that the product meets Regulation 722.411.4.1 in the specific way they are installing it, and that it is documented on the Electrical Installation Certificate.

Whichever method is used, the earth electrode resistance should be tested after installation and recorded on the certificate. A resistance to earth below 200 ohms is acceptable for most residential open PEN detection schemes, though lower is better.

When to call an electrician

If you are considering any EV charger installation, or if your existing charger was fitted without an earth electrode or open PEN detection device, call a Part P registered electrician. They can check your supply earthing type, install the appropriate protection and issue the correct documentation. Most charger installers will already know this -- if a quote does not mention earthing and open PEN protection, it is worth asking directly.